1. Field of the Invention
The present invention relates to an optical burst receiving apparatus.
Along with the development of multimedia technology, optical communication using optical fibers has been rapidly spreading. Particularly, while optical communication hithertofore had mainly spread at the trunk line side, in the future optical communication will mainly spread at the subscriber side for realizing "fiber to the home" (FTTH) systems. The "subscriber side" referred to here includes not only individual subscribers (users), but also local area networks (LANS) etc. Note that the "subscriber side" spoken of in the present invention may further include even a system performing communication among two or more computers.
In for example a passive optical network (PON) transmission system, the subscriber side is constituted by an optical splitter (star coupler) connected to the office side equipment via a common optical fiber transmission line and a plurality of units of subscriber side equipment individually connected to this optical splitter via individual optical fiber transmission lines. Here, downstream optical transmission from the office side equipment to each subscriber side equipment is carried out by continuously transmitting cell signals, while upstream optical transmission from the plurality of units of subscriber side equipment to the office side equipment is carried out by using time slots allotted to the units of subscriber side equipment in advance and transmitting the cell signals of each subscriber in a burst-like manner in a time division mode in units of cells each having a fixed bit length.
Looking at the latter upstream optical transmission, the office side equipment receives the bursts of cell signals from the optical transmission apparatuses of the units of subscriber side equipment. The present invention relates to an optical burst receiving apparatus which receives the burst-like cell signals in the office side equipment. This optical burst receiving apparatus receives cell signals having different levels of received light for every time slot due to the difference of transmission loss of the individual optical fiber transmission lines. Therefore, the automatic threshold control (ATC) and automatic gain control (AGC) for this become important subjects.
2. Description of the Related Art
As an optical burst receiving apparatus in office side equipment used in for example the above PON transmission system, optical burst receiving circuits based on the following methods of control i) and ii) have already been disclosed:
i) The first is a high speed ATC/AGC method utilizing a high speed level-detecting circuit and PA1 ii) The second is a system ATC/AGC method utilizing sequence control information as one type of principal information in a PON transmission system.
As will be explained in detail later by referring to the drawings, in the system ATC/AGC method, both of a guard time (GD) and overhead region (OH) are small. Accordingly, the transmission efficiency of the entire PON transmission system is extremely good. This is a major advantage not obtained in the high speed ATC/AGC method.
However, there is also a disadvantage in this system ATC/AGC method. This is the occurrence of a portion of the signal wherein discrimination of the logic by the threshold value (TH2) becomes difficult due to the low frequency response. This causes the problem of failed reception. Further, even if reception does not fail, there is the problem of deterioration of the bit error rate (BER).